legacy ls: only output appropriate directories/containers

For lxc-ls without --active, only output a directory in lxc_path if it
contains a file named config. This avoids extra directories that may
exist in lxc_path, for example .snapshot if lxc_path is an nfs mount.

For lxc-ls with --active, don't output . if there are no active
containers.

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>